In a chromatographic mass spectrometer, a combination of a gas chromatograph (GC) or a liquid chromatograph (LC), and a mass spectrometer, various components contained in a sample to be tested are temporally separated through a column, and ions generated from the separated components are separated according to their mass-to-charge ratios m/z through a quadrupole mass filter or the like to be detected by a detector.
To make a quantitative determination on a known compound contained in a sample using such a chromatographic mass spectrometer, in general, an ion characterizing the compound is determined to be a target ion, and the target ion is subjected to selected ion monitoring (SIM) measurement or multiple reaction monitoring (MRM) measurement by the mass spectrometer. Then, based on data obtained from the measurement, an extracted-ion chromatogram (mass chromatogram) for the target ion is created, and the concentration of the target compound is calculated from the area, the height or the like of a chromatogram peak appearing at around the retention time of the target compound on the chromatogram. The target ion may also be referred to as a quantitative ion, and as such, an ion corresponding to a peak of a maximum signal intensity on a typical mass spectrum of the compound is normally selected.
Although a target ion is an ion characterizing a compound, various impurities may be included in an actual sample, or inappropriate separation conditions in a previous-stage chromatograph may result in insufficient component separation, making a plurality of compounds overlap one another. In such cases, only checking a chromatogram peak of a target ion having a specified mass-to-charge ratio MT makes it difficult to clearly determine whether or not the peak originates from a target compound. Therefore, in quantitative analysis using a chromatographic mass spectrometry, in general, an ion that characterizes the compound but has another mass-to-charge ratio MC is selected as a qualifier ion in addition to a target ion, and using intensity ratio between the signal intensity of the peak of the qualifier ion and the signal intensity of the peak of the target ion on a mass spectrum in actual measurement (hereafter, referred to as “qualifier ion ratio”), the confirmation of the target ion truly originating from a target compound, that is, the identification of the target ion is performed (see Patent Literature 1 and Non Patent Literature 1). In addition, for example, in the case where a plurality of compounds similar in structure are possibly contained in a sample, only one kind of qualifier ion may be insufficient to correctly identify a target ion of a certain compound, and thus a plurality of qualifier ions are often used for one compound.
In the case of simultaneous multiple-component analysis, quantitative determination may be performed on a large number of compounds as many as several tens, or several hundreds in some cases, at one time of chromatographic mass spectrometry. However, it is difficult for an operator to visually determine whether or not qualifier ion ratios are appropriate for such a large number of compounds. Thus, a conventional chromatographic mass spectrometer includes a determined identification range having a tolerance preset for each qualifier ion ratio and is configured to automatically execute a process in which a target ion is identified as an ion originating from a target compound when qualifier ion ratios in actual measurement fall within respective identification ranges (see Patent Literature 2, etc.).
FIG. 8 is a diagram illustrating a parameter setting screen for qualifier ions used to execute such an automated process in the conventional chromatographic mass spectrometer, and FIG. 9 is a diagram illustrating an advanced setting screen for qualifier ions for each target ion.
A method for identifying a target ion using qualifier ions includes two modes: “ABSOLUTE TOLERATION” and “RELATIVE TOLERATION”. The absolute toleration is a mode in which an identification range used to identify a target ion as one originating from a target compound is specified literally in the form of the absolute value of a qualifier ion ratio, and assuming that the qualifier ion ratio is denoted by Ri[%], and the tolerance is denoted by Rw[%], an identification range Pa is defined as follows.Pa=Ri±Rw[%]  (1)
Meanwhile, the relative toleration is a mode in which an identification range is specified in the form of the relative ratio of a qualifier ion ratio, and assuming that the qualifier ion ratio is denoted by Ri[%], and the tolerance is denoted by Rw[%], an identification range Pr is defined as follows.Pr=Ri±(Ri×Rw)/100[%]  (2)
In the qualifier ion parameter setting screen illustrated in FIG. 8, one of “ABSOLUTE TOLERATION” and “RELATIVE TOLERATION” is selectable as a qualifier ion mode by means of a drop-down menu, and when a qualifier ion ratio and a tolerance for a certain qualifier ion are given, an identification range is set according to the previously described Equation (1) or (2). Specifically, FIG. 9 is an advanced setting screen for qualifier ions illustrating the identification ranges of five qualifier ions for a target ion having a precursor ion m/z: 147.00, and a product ion m/z: 46.0 (m/z: 147.00>46.0) in MRM measurement. In this example, the qualifier ion mode is the absolute toleration, a default tolerance is ±30[%], and for example, for a qualifier ion having a qualifier ion ratio of 60.00, a precursor ion m/z: 130.00, and a product ion m/z: 41.0, an identification range from a lower limit of 60−30=30 to an upper limit of 60+30=90 is set. With this setting, for this qualifier ion, when a qualifier ion ratio based on an actually measured result falls within this identification range, a target ion of m/z: 147.00>46.0 is identified to originate from the target compound.
However, the conventional chromatographic mass spectrometer described previously involves the following problems. That is, while an operator can select either one of the absolute toleration and the relative toleration as the qualifier ion mode, the absolute toleration and the relative toleration may need to be combined for some purpose or application of measurement.
For example, the guidelines on drug test by Association of Official Racing Chemists (AORC), disclosed in Non Patent Literature 2, specify that a qualifier ion ratio in the absolute toleration and a qualifier ion ratio in the relative toleration are separately set, identification ranges in both modes are calculated using the separately set qualifier ion ratios, and whichever of an absolute tolerance and a relative tolerance is greater is employed as an identification range for the target ion identification. In addition, the previously described AORC guidelines specify that the upper limit of an identification range is 100% (i.e., specify that identification is disabled when the signal intensity of a qualifier ion exceeds the signal intensity of a target ion).
Meanwhile, with the conventional chromatographic mass spectrometer described previously, target ion identification consistent with such guidelines cannot be automatically performed, and thus the operator has to confirm whether or not a qualifier ion ratio is within an identification range, which requires very cumbersome operations. In addition, a qualifier ion ratio cannot be separately set for the absolute toleration and the relative toleration, and thus the operator has to set an identification range for the absolute toleration and an identification range for the relative toleration one by one. In addition, the operator has to modify identification ranges exceeding 100% one by one, which is very troublesome.
In addition, according to the identification criteria for pesticide residues in EU disclosed in Non Patent Literature 3, a recommended tolerance differs by qualifier ion. To set such a recommended tolerance in the conventional chromatographic mass spectrometer described previously, the operator has to confirm a qualifier ion ratio set for each qualifier ion and set a tolerance corresponding to the qualifier ion ratio, which is very troublesome.